1. Field of the Invention
The invention relates in general to elevator systems, and more specifically to traction elevator systems of the type having a dual-bridge converter, solid state DC motor drive.
2. Description of the Prior Art
The dual-bridge solid state converter constructed of thyristors has become an attractive alternative to the motor-generator set in supplying direct current for the DC drive motor of a traction elevator system. This solid state motor drive (SSMD) requires very little maintenance, since there are no moving parts, brushes, bearings, etc., and it is energy efficient, returning electrical energy to the AC supply lines during braking, and requiring no power when the elevator car is idle. The motor-generator set, however, does have an advantage when short power disturbances and interruptions occur while an elevator car is running. The inertia of the motor-generator set enables it to simply ride over short power interruptions and disturbances in a three-phase power source. With a SSMD, on the other hand, a short power interruption or voltage waveform disturbance may cause the firing circuits which gate the thyristors to gate the wrong thyristor of a bank, or to gate a thyristor of one bank while current is still flowing in the other bank. Incorrect firing of a thyristor can cause a line-to-line short circuit which blows the fast acting power fuses which protect the thyristors.
Monitoring and protective circuits have been developed to detect those power supply conditions which may result in fuse blowing, with the protective circuit automatically taking a predetermined corrective action to prevent fuse blowing. U.S. Pat. Nos. 4,155,427; 4,286,222 and 4,307,793, which are assigned to the same assignee as the present application, are directed to different aspects of power supply monitoring for elevator systems which utilize SSMD.
While the monitoring and protective circuits of the above-mentioned patents operate satisfactorily, fuse blowing has still occurred sporadically over the years for reasons which have not been entirely understood. For example, fuse blowing has occurred in the drive of a moving elevator car when the electrical power is switched from a main feeder to an auxiliary feeder, and also when the electrical power is switched from a main feeder to an emergency feeder, with fuse blowing occurring regardless of the switching speed. Fuse blowing in the SSMD of a moving elevator car has also occurred when storms have blown ice covered transmission phase conductors together, suddenly and momentarily dropping transmission voltages from the normal level to a lower voltage level.